Gaseous hydrogen fluoride is evolved in several industrial processes, the most important of which are aluminium smelting and enrichment of uranium via uranium hexafluoride. As hydrogen fluoride is a serious atmospheric pollutant, it is important that the amount of hydrogen fluoride released into the atmosphere is monitored to control its release in such processes. Gaseous hydrogen fluoride may also be generated by power stations and in brickworks.
Whilst the following description is directed to monitoring hydrogen fluoride evolution in the aluminium smelting industry, it will be apparent that the method and apparatus of the present invention can be employed to detect and determine gaseous hydrogen fluoride in any medium with significant transmission at the appropriate frequencies.
Present proposals for aluminium smelting in the Hunter Valley of N.S.W. envisage a large increase in aluminium output ("Pollution control in the Hunter Valley with Particular Reference to Aluminium Smelting" N.S.W. State Pollution Control Commission (hereinafter referred to as SPCC) Report, August 1980).
In the production of aluminium, alumina powder is electrolysed in molten cryolyte (Na.sub.3 AlF.sub.6) to aluminium metal and oxygen which combines with the carbon of the carbon anodes to produce carbon monoxide and carbon dioxide. During the process about 20 kg of fluoride, mainly in the form of gaseous hydrogen fluoride is emitted per tonne of aluminium produced.
Aluminium production is carried out in long pot-rooms, which are typically 800.times.25 m and which each generally contain about 100 electrolytic cells. Hoods over the electrolytic cells draw off approximately 97.5% of the emitted gaseous hydrogen fluoride and exhaust it through dry scrubbers which retain 99.9% hydrogen fluoride, thence to stacks. The main source of fluoride pollution is gaseous hydrogen fluoride escaping from the cell hoods, particularly when anodes are changed. The gaseous hydrogen fluoride is released from the roof vents of the pot-room as described in the SPCC report, August 1980 supra. The maximum concentration allowed in the pot-rooms on the ground of industrial health is 2 mg.m.sup.-3. The SPCC have imposed an obligation on the smelter operators to monitor continuously the gaseous hydrogen fluoride concentration in the pot-rooms with levels in the range 40-400 .mu.g.m.sup.-3 as a suggested working range. Monitoring gaseous hydrogen fluoride conveniently and economically has heretofore posed many problems.
Present monitoring methods involve pumping air containing gaseous hydrogen fluoride through heated filters to remove fluoride particulates and then through solutions or lime beds. The calcium fluoride formed is assayed using a fluoride selective electrode. These methods are said to be inconvenient, not suitable for reliable automation, and not very accurate.
Whilst not required at present, it is proposed that the SPCC will require aluminum smelter operators to monitor continuously the scrubber stack emmissions, and the gaseous hydrogen fluoride concentrations at four positions inside and two positions outside the plants. Permissible gaseous hydrogen fluoride levels outside the pot-rooms may be in the range 1-10 .mu.g.m.sup.-3.
Atmospheric pollutants may be monitored by long path length optical absorption measurements as described in "Troposcopic Photochemical and Photophysical Processes", J. N. Pitts and B. J. Finlayson-Pitts; "Remote Sensing Using Tunable Lasers", K. W. Rothe and H. Walter; in "Tunable Lasers and their Applications," ed. A. Mooradian et al, Springer-Verlag 1976. When laser light sources are employed, ranges of up to 10 km are possible. With optical radar techniques using topographic reflectors or aerosol scattering it is possible to range gaseous absorbers up to a few kilometers from the light source.
The concentrations of gaseous hydrogen fluoride can be monitored using its infra-red absorption. Gaseous hydrogen fluoride molecules are usually in the ground state (no vibrational excitation) and may sequentially absorb light at wavelengths corresponding to transitions to the first, second, third vibrational level etc. These 0.sup..fwdarw. 1, 1.fwdarw.2, 2.fwdarw.3, transition frequencies are further split into rotational absorption lines specified as P or R branch transitions. In this specification, the usual convention of describing 0.fwdarw.1 transitions as 1P or 1R lines and 1.fwdarw.2 transitions as 2P or 2R lines is adopted. Transitions such as 0.fwdarw.2 or 0.fwdarw.3 are described as overtones and are very low in intensity. The absorption frequencies characteristic of gaseous hydrogen fluoride have been extensively reported, e.g. R. J. Lovell and W. F. Herget, J. Opt. Soc. Am., 52, 1374 (1962), W. F. Herget et al., ibid., 52, 1113 (1962), D. E. Mann et al., J. Chem. Phys., 34, 420 (1961), D. F. Smith, Spectrochimica Acta, 12, 224 (1958), and G. Guelachvili, Optics Commun., 19, 150 (1976). It has been reported that the absorption peaks are very much broadened and slightly shifted when mixed with gases at atmospheric pressure by J. J. Hinchen, J. Opt. Soc. Am., 64, 1162 (1974) and B. M. Shaw and R. J. Lovell, J. Opt. Soc. Am., 59, 1598 (1969).
Previous optical studies of gaseous hydrogen fluoride pollution have employed the well-known non-dispersive infra-red analysis (hereinafter referred to as NDIR) technique (W. F. Herget et al., Applied Optics, 15, 1222 (1976)), and a Ga-As diode laser absorbed by the 0.fwdarw.3 overtone (V. B. Anzin et al., Soviet J. Quant. Electron. (U.S.A.) 5, 754 (1975)). With the NDIR technique, a path length greater than about 10 m is not feasible and the above paper on NDIR suggests that the sensitivity could be approximately 500 .mu.g.m.sup.-3 gaseous hydrogen fluoride in a path length of 10 m. The technique did not detect levels of 100 .mu.g.m.sup.-3, though it could be of interest for stack gas monitoring. The Ga-As diode laser technique uses kilobar hydrostatic pressures to tune the diode, is difficult to control in frequency, and is not sensitive.
Tunable diode lasers, tuned by current and temperature, are not yet available at frequencies higher than 3750 cm.sup.-1 and there has been no extension of their availability in this area in the past six years. They do not yet cover the range required for gaseous hydrogen fluoride monitoring (3500-4500 cm.sup.-1). Other tunable lasers are available: the colour-centre lasers and the spin-flip raman lasers. They are short-lived and require such complexities as nitrogen or helium cryostats and superconducting magnets. All tunable lasers, moreover, would involve formidable problems of stability and resetability.
A hydrogen fluoride laser would be the most unequivocal source for monitoring gaseous hydrogen fluoride through its infra-red absorption. The hydrogen fluoride laser is a chemical laser. Fluorine and hydrogen atoms are generated in a glow discharge (e.g. through sulfur hexafluoride and propane) and combine to form vibrationally excited gaseous hydrogen fluoride which, in a cw laser, then lases on its 3.fwdarw.2, 2.fwdarw.1, 1.fwdarw.0 vibration levels emitting several of the P rotational lines from each level. Of these lines, only the 1P lines are absorbed by gaseous hydrogen fluoride at normal temperatures.
Most reports concerning the atmospheric transmission of gaseous hydrogen fluoride laser outputs are for military applications, e.g. "Handbook of Chemical Lasers", ed. R.W.F. Gross and J. F. Bott, Wiley, 1976, and "Topics in Current Chemistry: Vol 37 Chemical Lasers", K. L. Kompa, Springer-Verlag, 1973. Some reports have referred to atmospheric pollution measurements, e.g., A. S. Gomenyuk et al., Soviet J. Quant. Electron. (U.S.A.), 4, 1001 (1975) and K. W. Rothe and H. Walter, supra. Only one report, A. Tonnissen et al., Applied Physics (Springer-Verlag) 18, 297 (1979), relates to long path gaseous hydrogen fluoride absorption monitoring with a hydrogen fluoride laser.
Tonnissen et al. reported the use of a cw hydrogen fluoride laser to monitor the gaseous hydrogen fluoride concentration in the cell room of the Vereinigten Aluminium Werke smelter. The laser was untuned and lased simultaneously on the 1P (4 to 7) lines and on several 2 P lines. The 2 P lines are not absorbed by gaseous hydrogen fluoride and were used to determine the attenuation due to particulate scattering, carbon dioxide and water vapour. The authors' main problem was the absorption due to water vapour and, to a lesser extent, carbon dioxide. Tonnissen et al., monochromated the output on each line in turn and measured transmission using the usual technique of a chopper and lock-in amplifier, with a retro-reflector at approximately 100 m range. The concentrations of gaseous hydrogen fluoride, and of water vapour, carbon dioxide and particulates, were determined by solving the simultaneous equations with coefficients determined by laboratory measurements of the cross-sections of gaseous hydrogen fluoride, water vapour and carbon dioxide.
The lines available from the type of laser used by Tonnissen et al. are far from ideal regarding their freedom from interference by water vapour. This can be seen with reference to FIG. 3-16 at page 67 of "Infrared Physics and Engineering" J. A. Jamieson et al McGraw-Hill 1963. Tonnissen et al employed lines at 3644.24, 3693.21, 3741,36, 3788.13 and 3833.66 cm.sup.-1. Most of the precision of their measurement would probably have come from the 3788.13 cm.sup.-1 line alone. Nevertheless they claimed a precision of 200 .mu.g.m.sup.-3 which would almost meet the SPCC requirements.
The major drawbacks of employing a gaseous hydrogen fluoride laser to monitor the concentration of hydrogen fluoride are:
(i) A gaseous hydrogen fluoride laser requires continuous pumping and a continuous supply of e.g. helium, hydrogen and sulfur hexafluoride; PA1 (ii) Many of the gaseous hydrogen fluoride laser lines suffer excessive interference from water vapour and carbon dioxide; PA1 (iii) The whole system would pose formidable problems in engineering optimisation and maintenance. PA1 (i) a first source of laser light at a first frequency corresponding to a neon 3 p.sub.4 -2 s.sub.2 transition; PA1 (ii) first detecting means associated with said first source, located remote from said first source by a suitable path length and being capable of detecting the intensity or absorption of said laser light at said first frequency; PA1 (iii) a second source of laser light at a second frequency excluding said first frequency, preferably said second frequency is between 4753 cm.sup.-1 and 4000 cm.sup.-1 and most preferably between 4173 cm.sup.-1 and 4176 cm; PA1 (iv) second detecting means associated with said second source, located remote from said second source by a suitable path length and being capable of detecting the intensity or absorption of said laser light at said second frequency; and PA1 (v) calculating means associated with the outputs of said first and second detecting means to calculate said concentration;
Fourier transform infra-red spectrometers ("Introduction to Fourier Transform Spectroscopy" R. J. Bell, Academic Press, 1972) can provide adequate high resolution absorption spectra over long path-lengths but are complex and expensive instruments not suited to rapid data acquisition.
The present invention is based upon the discovery by the present inventor that the helium-neon laser can be employed to generate an infra-red laser beam at a frequency which very nearly coincides with one of the absorption lines of gaseous hydrogen fluoride.
FIG. 15 in "Remote Fourier Transform Infra-red Air Pollution Studies", W. F. Herget and J. D. Brasher, Opt. Eng., 19, 508 (1980) shows a transmission spectrum between 4173 and 4176 cm.sup.-1 of clean air and of air containing gaseous hydrogen fluoride from a gypsum pond. The strength of the water vapour line at 4174.67 cm.sup.-1 in the clean air spectrum was matched with that of the same line in the spectrum of the air containing gaseous hydrogen fluoride and the spectra were subtracted to eliminate the effect of the weak water vapour line interference. This revealed a substantially flat base having a peak at a frequency observed to be 4173.9798 cm.sup.-1 by G. Guelachvili, Optics Communications, 19, 150 (1976) superimposed thereon. The instrumentally broadened peak width observed by Herget and Brasher was 0.15 cm.sup.-1 full width half maximum, and the substantially flat base line extended from 4173 cm.sup.-1 and 4176 cm.sup.-1.
In a helium-neon laser using .sup.20 Ne, it follows from the energy levels reported by C. E. Moore in "Atomic Energy Levels", Circular of NBS No. 467, (U.S. Government Printing Office Washington DC 1949, page 77) that the 3 P.sub.4- 2 s.sub.2 (Paschen Notation) transition emits at 4174.01 cm.sup.-1 and that a 3 p.sub.2 -2 s.sub.2 transition emits at 4173.13 cm.sup.-1.
Although Guelachvili, supra, and Moore, supra, taken together would indicate that the mis-match between the gaseous hydrogen fluoride 1 R5 line and the .sup.20 Ne 3 p.sub.4 -2 s.sub.2 He-Ne laser line at 4174.01 cm.sup.-1 was 0.031 cm.sup.-1 , it has been measured by the present inventor and found to be less than 0.01 cm.sup.-1. The pressure broadening of the gaseous hydrogen fluoride 1 R5 line by various gases at atmospheric pressure has been reported by R. Beigang et al., Physical Review A 20, 299 (1979) at approximately 0.1 cm.sup.-1 full width half maximum. Width of the same order would be expected for other media and the resultant pressure broadening of the gaseous hydrogen fluoride absorption line reduces the effect of pressure induced shift of the centre frequency of the gaseous hydrogen fluoride line.
The apparatus and method of the present invention have the major advantage of being able to use helium-neon tubes incorporating well established commercial technology which are filled with any suitable neon isotope.
The apparatus and method of the invention can be operated on any isotope of neon or on mixtures of neon isotopes.